This invention relates to an improved method of ultrasonically consolidating layers or plies of fiber-reinforced polymer-matrix composites, and more particularly to such a method which applies the ultrasonic energy generally parallel to the surface layer to produce substantial shear in the material to effect heating. The invention also relates to the product made by that method.
Composite materials are becoming more and more attractive for a wide variety of uses, from aircraft and automobiles to sporting goods and toys, because of their high stiffness and strength-to-weight ratio. A composite material is a combination of fibers in a matrix or resin. Typically, a composite structure is made of a number of layers or plies of composite material. As used herein, a composite material means a structure composed of a plurality of plies of fiber-reinforced fabric or tape in a resin matrix. Dry fabric with unidirectional fibers or woven fibers is often precombined with resin as a xe2x80x9cPREPREGxe2x80x9d. Examples include glass or graphite fibers in a xe2x80x9cPEEKxe2x80x9d matrix. The fibers typically comprise more than 35% of the material. One type of composite, a thermoset composite, requires that the fiber/resin plies be laid-up, and then the composite heated to cure it. This can take a matter of hours.
As the demand for composites increases, so has the demand for faster fabrication techniques. Another type of composite material, thermoplastic composites, are faster to fabricate because there is no curing involved. The thermoplastic ply need only be heated to melt the plastic matrix, then pressed together or consolidated to the previous ply before cooling. Consolidation means laminating two or more plies together to form a part or structure. Good consolidation implies a low level of entrapped voids (typically less than 3%) and a shear strength of the ply to ply interface which approaches that of the resin matrix.
Heating the plies, however, is troublesome: a number of different heating techniques have been tried but have met with mixed success. Laser heating in the nip between the previous layer and the one being applied has not been wholly successful due to the practical problems by applying the energy at the nip. A laser apparatus with all necessary controls is also quite expensive. Infrared devices, which depend upon radiant heating, suffer from poor heat modulability and can easily damage the composite. Microwave devices suffer similar shortcomings and are potentially hazardous to surrounding personnel as well. A hot shoe technique which is commercially available uses a series of massive, heated iron-like devices. This method relies on conduction through the ply to heat the interface which makes the process a slow one. Because of size and geometry this method has only been applied to the production of flat panels, thereby restricting its usefulness. The most evaluated technique presently in use is hot gas heating. In that process a stream of hot air or gas is aimed into the nip between the new ply (layer or tape or tow) and the substrate, following which the ply is pressed or ironed onto the substrate using a pinch roller or a shoe. While the consolidation levels achieved are high the heating is difficult to modulate with respect to rapid changes in the material feed rate. This complicates the practical integration of convective, hot gas, heating with standard computer-numerical-control fabrication equipment. Moreover, despite the high consolidation some reports on the mechanical properties of the resulting composites have been disappointing. This may be due to damage or degrading of the surface of the material at the nip, especially due to the high heat applied and the large temperature differential (300xc2x0 C. or more) between the hot gas stream and the melt temperature of the thermoplastic material.
Ultrasonic devices used to heat the plies have appeal for a number of reasons. Unlike convection (hot gas), conduction (hot shoes/irons), or radiation (infrared) ultrasonics does not depend upon a thermal driver to effect energy transfer to the composite material. Ultrasonic heating is instantaneously modulatable, and it provides deep, penetrating heating in the plastic matrix beyond mere surface heating.
Ultrasonic welding has long been used to weld or bond neat (unreinforced) plastics. Such welding is done by placing the horn perpendicular to two plastic layers, pressing down on the layers and energizing the horn. Obeda, U.S. Pat. No. 4,713,131, teaches joining large sheets of polypropylene plastic by overlapping the sheets of plastic and welding their edges together using an ultrasonic horn placed between the sheets. Obeda teaches nothing about composite materials.
But, others have attempted to use an ultrasonic horn to fabricate composite parts. See Joining Methods for Plastic and Plastic Composites: An Overview, Vijay Stokes, Polymer Engineering and Science, Mid-October 1989, Vol. 29, No. 19, p. 1310-1324, see specifically pp. 1322-1324, items 168-236. These previous attempts to weld thermoplastic composites during the lamination process using conventional ultrasonic welding have yielded disappointing results because, it is speculated, the presence of the fibers alters the energy transfer in the material. The conventional ultrasonic welding technique sets up a compression wavefront in the material which does not transmit well through the material. In 1987, engineers at Martin Marrietta attempted to use an ultrasonic horn to consolidate composite resin-fiber plies. The horn was placed on the top of two moving plies to be consolidated in a direction perpendicular to the plies. A range of different pressures, energy levels, and feed rates were tried. The result, however, was not satisfactory: xe2x80x9cC-Scan results have shown that attempts to produce consolidated or near-consolidated laminates have not been successful thus far . . . xe2x80x9d Sonic Assisted Process Developmentxe2x80x9d, Interim Technical Report,xe2x80x9d contract No. F 33615-86-5041, Martin Marrietta Baltimore for Material Laboratory Air Force Wright labs., March 1987.
Therefore, although ultrasonic horns have successfully been used to weld plastic sheets together and, to some extent, have been successfully used to weld plastics containing up to about 35% filler (such as glass or talc), the state of the art reveals no successful methodology of fabricating fiber matrix composite structures wherein an ultrasonic horn is used to consolidate the individual fiber-resin plies.
It is therefore an object of this invention to provide a method of fabricating a fiber matrix composite structure.
It is a further object of this invention to provide such a method which utilizes a ultrasonic horn to consolidate the fiber resin plies of the composite structure.
It is a further object of this invention to provide such a method which is controllable, instantly modulatable, and which does not require a large thermal differential between the device and the material.
It is a further object of this invention to provide such a method which is much less likely to cause overheating or damage to the material or detract from the consolidation quality.
It is a further object of this invention to provide such a method which applies heat and pressure simultaneously.
It is a further object of this invention to provide such a method which is faster and easier to employ and is less expensive both in execution and in the equipment required, and is extremely energy-efficient.
The invention results from the realization that instead of orientating an ultrasonic horn perpendicular to the resin-fiber plies during fabricating which fails to provide consolidation, if the ultrasonic horn is orientated at an acute angle to the surface of the plies of resin-fiber material so that the horn motion is generally parallel to the laminate surface, and shear force is created in the plies which heats and fully consolidates the plies.
This invention features a method of fabricating a fiber matrix composite structure and may suitably comprise, include, consist essentially of, or consist of the steps of assembling a stack of plies of fiber-reinforced polymer-matrix material on a mandrel; engaging an ultrasonic horn with the top surface of uppermost ply; and orienting the horn at an acute angle with respect to the top surface and energizing the horn to induce a shear force in the plies to heat and consolidate the plies forming a composite structure.
The plies typically comprise at least 40% fiber a consolidation force is usually applied to the plies. The consolidated force may be applied through the horn or proximate the horn. Preferably, the acute angle is less than or equal to 15 degrees.
The fiber matrix structure may be a thermoplastic polymer-matrix or a thermosetting polymer-matrix. This invention also features a composite structure made by the aforedescribed method.